1. Field of the Invention
The present invention relates to an electrode material for an electrochemical capacitor, an electrochemical capacitor comprising an electrode containing such an electrode material, and a method for the production of such an electrode material.
2. Prior Art
Electric double layer capacitors utilize the capacitance of an electric double layer formed at an interface between a solid and a solution. In general, an electric double layer capacitor comprises a pair of polarizable electrodes (positive and negative electrodes), an electrolyte impregnated in the electrodes, a porous separator which contains an electrolyte impregnated therein, is ion-permeable and has electrical insulation properties so that it separates the electrodes to prevent the formation of a short circuit between them, and collectors connected to the respective electrodes. For example, in the case of a coin-type capacitor, a pair of electrodes, a separator interposed between the electrodes, and an electrolyte are stored in a metal case (can), and the case is sealed with a metal lid through an electrically insulating gasket to prevent the leakage of the electrolyte.
Such electric double layer capacitors have intermediate properties between properties of cells and those of electrolytic capacitors, and have a large capacitance with a small volume. Thus, in these years, electric double layer capacitors attract attentions as the backup sources of small-sized electric devices, etc.
Electric double layer capacitors comprise polarizable electrodes as positive and negative electrodes, as described above. Hitherto, activated carbon having a large surface area is used as the material of such polarizable electrodes, since it has a large specific surface area and carries a large amount of electric charge on its surface, and thus the capacitance of electric double layer capacitors increases. In general, electrolytes comprise a medium having a high dielectric constant such as water or carbonates (e.g. propylene carbonate, etc.) to dissolve solutes in a high concentration.
However, the maximum specific surface area of activated carbon is practically about 3000 m.sup.2 /g. Thus, the capacitance per unit volume of electric double layer capacitors comprising electrodes containing activated carbon almost reaches its limit. Therefore, electrochemical capacitors are proposed, which comprise electrodes containing rutile-type ruthenium oxide, amorphous hydrated ruthenium oxide, or their hydrogenated products (e.g. RuO.sub.2.multidot.xH.sub.2 O, etc.), which have a larger capacitance than activated carbon. However, electrochemical capacitors having electrodes containing such ruthenium oxides have a problem such that material costs are high since ruthenium is a rare material, although they have a ten to fifty times larger capacitance than those having electrodes containing activated carbon.